The present invention relates to a process for preparing the powder of raw ceramic material having fine crystal grain sizes.
As the size of electronic devices becomes increasingly smaller, efforts are being made to reduce the size of components to be incorporated in such devices. An example of such components is a ceramic capacitor, and since it is impossible to achieve a significant increase in dielectric constant by the state-of-the-art technology, the only way to reduce the size of a ceramic capacitor is by decreasing its thickness. However, if the thickness of a ceramic capacitor is simply reduced, its dielectric loss is increased and a greater change in capacitance occurs as a result of changes in DC or AC bias. In particular, if the crystal grain size of the ceramic is as great as 8 .mu.m, relatively large vacancies (some may be as large as 20 .mu.m) occur between grains and this causes a drop in the breakdown voltage of the capacitor.
In order to reduce the thickness of a ceramic capacitor without having these disadvantages, the capacitor must be made of a ceramic having a finer crystal grain size. As the crystal grain size is reduced, the vacancies occurring between grains become smaller so as to improve the breakdown voltage of the capacitor. Desirably, the grain size should be close to 0.6 .mu.m which is the thickness of the 90.degree. domain wall. This reduces the occurrence of a cubic to tetragonal transformation during a firing step in the state of lowering temperatures. This also prevents the capacitance of the device from dropping as a function of time (aging). As a further advantage, the ratio of the c- to a-axis of the ceramic crystal approaches 1.00 to reduce the change in capacitance resulting from variations in DC or AC bias. An additional advantage is improved mechanical strength.
Conventionally, powders of raw ceramic materials are prepared by the solid-phase reaction technique using dried raw materials such as BaCO.sub.3, CaCO.sub.3, SrCO.sub.3, TiO.sub.2, ZrO.sub.2 and SnO.sub.2. One synthesis method starts with calcination of raw materials such as BaTiO.sub.3 and CaZrO.sub.3, which are mixed and subsequently fired. In another synthesis method, two or three raw materials selected from the group BaCO.sub.3, CaCO.sub.3, SrCO.sub.3, TiO.sub.2 and ZrO.sub.2 are mixed in given proportions and the mixture is calcined. However, either method has one serious defect that is attributable to the use of dried BaCO.sub.3, CaCO.sub.3, SrCO.sub.3, TiO.sub.2 and ZrO.sub.2 as raw materials. These materials are first subjected to a precipitation step to obtain fine colloidal particles, but when they are dried (and subsequently calcined if desired) after filtration, the particles agglomerate to form secondary grains having sizes between 0.5 and 2 .mu.m. A blend of two or more raw materials comprising such agglomerated secondary particles cannot be ground to a size smaller than 1 .mu.m. When such a blend is formed into a suitable shape and sintered, the resulting product has crystal grains as large as 8 to 20 .mu.m and suffers the problems mentioned above (i.e., low breakdown voltage, and variations in capacitance with time, as well as with changes in AC and DC bias).
In order to produce a fine-grained ceramic, raw materials comprising fine colloidal grains as obtained by precipitation must be mixed. This can be realized by either the oxalate or alkoxide method. According to the oxalate method, ions such as Ti and Ba ions are reacted with oxalic acid to precipitate barium titanyl oxalate [BaTiO(C.sub.2 O.sub.4).4H.sub.2 O]; the precipitate is thermally decomposed to BaTiO.sub.3 at a temperature higher than 700.degree. C. While the fineness of the grains obtained by this method is fairly satisfactory, no elements other than Ba and Ti can be precipitated simultaneously. Therefore, the oxalate method is unable to produce commercially useful multi-component ceramics. Another disadvantage that makes the oxalate method unsuitable for practical purposes is the high cost of the oxalate formed.
The alkoxide method involves difficulty in preparing alkoxides of various metals, and the alkoxides that could be obtained are very expensive. Another factor that reduces the commercial value of this method is the use of organic solvents because protection must be provided against explosion of solvent vapor.
Both oxalate and alkoxide methods depend on reactions in solutions, but in the absence of a crystal growth inhibitor, the grains grow to sizes between about 1 and 2 .mu.m during sintering. These methods provide fine primary particles but since they cannot be agglomerated to larger secondary particles, a large amount of binder must be used to agglomerate the fine primary particles. But then, the shaped material shrinks so greatly during firing that the desired ceramic product cannot be produced.
In short, the conventional techniques for preparing raw ceramic materials are defective in that they satisfy none of the requirements mentioned below or they satisfy only some of those requirements:
(1) all the components for making a composite system are precipitated by a reaction in solution, and the growth of crystal grains can be restrained as required;
(2) the respective raw materials can be mixed in solution;
(3) the fine primary grains can be agglomerated to secondary particles in order to reduce the use of a binder in the forming step; and
(4) the desired raw ceramic materials can be prepared safely and at low cost.
Therefore, the principal object of the present invention is to provide a process for preparing fine-grained raw ceramic materials without suffering from the defects of the conventional techniques.